wrlib/alpha_combine.c

   1 /* alpha_combine.c - Alpha channel combination, based on Gimp 1.1.24
   2  * The GIMP -- an image manipulation program
   3  * Copyright (C) 1995 Spencer Kimball and Peter Mattis
   4  *
   5  *  This library is free software; you can redistribute it and/or
   6  *  modify it under the terms of the GNU Library General Public
   7  *  License as published by the Free Software Foundation; either
   8  *  version 2 of the License, or (at your option) any later version.
   9  *
  10  *  This library is distributed in the hope that it will be useful,
  11  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13  *  Library General Public License for more details.
  14  *
  15  *  You should have received a copy of the GNU Library General Public
  16  *  License along with this library; if not, write to the Free
  17  *  Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  18  */
  19
  20 #include "wraster.h"
  21
  22 void RCombineAlpha(unsigned char *d, unsigned char *s, int s_has_alpha,
  23                    int width, int height, int dwi, int swi, int opacity) {
  24         int x, y;
  25         int t, sa;
  26         int alpha;
  27         float ratio, cratio;
  28
  29         for (y=0; y<height; y++) {
  30                 for (x=0; x<width; x++) {
  31                         sa=s_has_alpha?*(s+3):255;
  32
  33                         if (opacity!=255) {
  34                                 t = sa * opacity + 0x80;
  35                                 sa = ((t>>8)+t)>>8;
  36                         }
  37
  38                         t = *(d+3) * (255-sa) + 0x80;
  39                         alpha = sa + (((t>>8)+t)>>8);
  40
  41                         if (sa==0 || alpha==0) {
  42                                 ratio = 0;
  43                                 cratio = 1.0;
  44                         } else if(sa == alpha) {
  45                                 ratio = 1.0;
  46                                 cratio = 0;
  47                         } else {
  48                                 ratio = (float)sa / alpha;
  49                                 cratio = 1.0 - ratio;
  50                         }
  51
  52                         *d = (int)*d * cratio + (int)*s * ratio;
  53                         s++; d++;
  54                         *d = (int)*d * cratio + (int)*s * ratio;
  55                         s++; d++;
  56                         *d = (int)*d * cratio + (int)*s * ratio;
  57                         s++; d++;
  58                         *d = alpha;
  59                         d++;
  60
  61                         if (s_has_alpha) s++;
  62                 }
  63                 d+=dwi;
  64                 s+=swi;
  65         }
  66 }